Photodynamic therapy (PDT) is a cancer treatment modality in which cancer cells are destroyed through the energy represented by light photons. Typically, a light sensitive antibody made from cattle hemoglobin, called hematoporphyrin derivative (HpD), or a further purified mixture of HpD known as dihematoporphyrin ether/ester (DHE), is introduced into a patient's body. The antibody is purged from normal tissues during a latent period of 48-72 hours, but remains preferentially concentrated within malignant tissue. When the retained antibody is illuminated by light of a specific wavelength, it becomes excited and undergoes a spin conversion process in which energy is transferred to oxygen. The activated oxygen (known as singlet oxygen) is a highly reactive species which lethally damages cellular components - in this case, the surrounding malignant tissue
Studies indicate that to achieve maximum tissue penetration during PDT, the wavelength of the activating light energy should be close to 630 nm. However, no presently available laser produces light at this specific wavelength. Therefore, PDT is typically performed by using an argon laser to pump a dye laser, which in turn is tuned to produce light having the desired wavelength of 630 nm. This results in bulky equipment and large electric power requirements.
The 630 nm activating light is ordinarily delivered through an optical fiber to a diffuser at the tumor tissue However, the use of an optical fiber during PDT is undesirably intrusive. Moreover, the effectiveness of PDT using an optical fiber is reduced when cancer cells are distributed over large organ surfaces or when the cancer cells are located in crevasses. In such cases, cancer cells cannot be fully illuminated even when a diffuser is employed.
In an effort to overcome the above-noted drawbacks, a peroxylate-based chemiluminescent agent has been suggested as a possible light source for PDT. However, this agent requires initiation by admixing harsh peroxides which cannot be tolerated by normal tissues.